A measurement device (linear scale) which includes a long scale frame with a built-in scale and a detector that detects an amount of relative movement with the scale, and which measures the distance of movement between a pair of measurement target parts in a machine tool or the like, is known. In such a linear scale, a first measurement target part in the pair of measurement target parts is fixed to the scale frame, and a second measurement target part is fixed to the detector. The linear scale measures the distance of movement between the measurement target parts by the detector reading the amount of relative movement with respect to the scale built into the scale frame.
Here, the scale frame is formed from aluminum, for example, while the first measurement target part is formed from iron, for example. The coefficient of linear expansion of aluminum is 23×10−6/K, whereas the coefficient of linear expansion of iron is from 11 to 12×10−6/K. When thermal expansion occurs due to changes in the ambient temperature of the linear scale and the like, the expansion/contraction of the scale frame (which is aluminum) will be restricted by the first measurement target part (iron) due to difference in the coefficients of linear expansion, which produces thermal stress.
If thermal stress arises, the scale frame will experience deformations such as expansion/contraction, bending, or twisting. Furthermore, if the scale frame deforms, the scale which is built into the scale frame will also deform by bending, twisting, or the like. If the scale deforms, a problem will arise in that the detector will detect the amount of relative movement with respect to the scale with reduced precision.
Here, “expansion/contraction” of the scale frame caused by heat refers to deformation in a direction parallel to the measurement direction of the linear scale. “Bending” of the scale frame caused by thermal stress refers to deformation in a direction perpendicular to the measurement direction of the linear scale.
Patent Document 1, for example, discloses a unit-type linear displacement measurement apparatus (a linear scale) that addresses the above-described problem. This apparatus includes a frame body (a scale frame), extending in a length measurement direction (a measurement direction) and containing a main scale (a scale), and end portion fixed members (fixed blocks) attached to both ends of the frame body in the length measurement direction.
The frame body includes frame body end portions, which are located on both ends in the length measurement direction and are machined so as to be capable of connecting with the end portion fixed members, and a plate spring mechanism configured to be capable of absorbing bending in the frame body produced by the connections between the end portion fixed members and the frame body end portions.
The plate spring mechanism includes a plate formed so as to extend from the frame body end portions in the length measurement direction, and is configured by connecting that plate to the end portion fixed members.
Each of the end portion fixed members include an attachment hole for attaching the frame body to a measurement subject (the first measurement target part) and a parallel plate spring mechanism configured to be capable of absorbing expansion/contraction arising in the frame body.
The parallel plate spring mechanism includes two plate-shaped members formed by providing cutouts and cavities in the end portion fixed member. The plate-shaped members extend in a width direction of the frame body, which is perpendicular to the length measurement direction. The parallel plate spring mechanism is constituted by these two plate-shaped members.
According to the unit-type linear displacement measurement apparatus, the plate spring mechanism absorbs bending in the frame body, and the parallel plate spring mechanism absorbs expansion/contraction of the frame body.